Optical communication systems employing wavelength division multiplexed (WDM) technology achieve large transmission capacity by spacing optical channels as closely as possible, typically less than a nanometer (nm) apart. As the channel spacing decreases, monitoring spectral characteristics of the channels becomes more critical in verifying system functionality, identifying performance drift, and isolating system faults. For example, such monitoring is critical in detecting wavelength drift, which can readily cause signals from one optical channel to cross into another. Also, real-time feedback to network elements is critical to ensure stable operation of optical amplifiers commonly employed in the network.
Optical instruments for measuring optical power as a function of wavelength, called optical spectrum analyzers (OSAs), are known in the art. Most conventional OSAs use a wavelength tunable optical filter, such as a Fabry-Perot interferometer or diffraction grating, to resolve the individual spectral components. In the latter case, light is reflected off the diffraction grating at an angle proportional to the wavelength. The spectrum of the light is then analyzed on the basis of the angle at which the light is diffracted using a detector array. Alternatively, the diffracted light is moved over a slit and then detected using a small detector.
Alternatively, a Fabry-Perot interferometer may be used consisting of two highly reflective, parallel mirrors that act as a resonant cavity, which transmits light only at a unique frequency (wavelength). Wavelength tuning may be accomplished by varying the mirror spacing or rotating the interferometer with respect to the incident light so as to provide an optical spectrum analysis.
Other OSAs known in the art are based on the Michelson interferometer, wherein the incident light is split into two paths. One path is fixed in length, and the other is variable so as to create an interference pattern between the signal and a delayed version of itself, known as an interferogram. The wavelength of the incident light can be determined by comparing the zero crossings in the interferogram with those for a known wavelength standard. The optical spectrum, however, is determined by performing a Fourier transform on the interferogram.
Traditional optical spectrum analyzers (OSAs) are manufactured as laboratory devices which have to operate under laboratorial environmental conditions. A sophisticated wavelength and optical power calibration from time to time is required to ensure the wavelength and power accuracy of the device. Furthermore, they are generally bulky as well as costly.
Optical communication systems require industrial grade optical performance monitors (OPM), which function similarly to the traditional OSA, but are however subject to stringent industrial requirements. They must be relatively inexpensive, compact in size, with the reporting power and wavelength accuracy nearly the same as laboratorial grade OSAs, however without requiring extra calibration during the lifetime of the device, and be capable of monitoring light at densely spaced frequency points with high wavelength resolution and high dynamic range.
It is advantageous to have an OPM capable of monitoring all channels in one optical band of an optical communication link. It is also advantageous to have an additional functionality of monitoring an optical to signal noise ratio (OSNR) for each channel, which requires monitoring not only individual channels, but also light between channels to estimate an optical noise level, thereby further increasing spectral resolution requirements for an OPM. Today's WDM networks may employ as many as ˜200 channels with 25 GHz spacing between the channels in one optical communication band of ˜5000 GHz, which would benefit from an OPM capable of monitoring at lest 200 frequency channels with 25 GHz spacing. Such an OPM could also be advantageously used in communication systems having 200 GHz, 100 GHz, and 50 GHz spaced channels by providing an OSNR monitoring capability.
Industrial-grade OPMs can be divided in two basic groups. The first one is based on tunable filters with output coupled to a photodetector, wherein the spectrum is measured by scanning the filter passband over a frequency range of interest, and adjacent spectral points are accessed sequentially in time. The tunable filter employed in this approach can be based on a bulk—surface or volume—grating, a fiber Bragg grating, a tunable linear or ring resonator. The second group of OPMs acquires all monitored spectral points of an optical spectrum of an input signal in parallel by dispersing the input light in space and using a plurality of photodetectors, e.g. a photodetector array, to simultaneously acquire spectral information at a plurality of monitored frequencies; a bulk grating, a blazed fiber Bragg grating, or an array waveguide grating can be used as such a dispersive element.
Both these approaches have their advantages and disadvantages. Using tunable filters may require complex dynamic control loops and real-time monitoring of the tuning to ensure reproducibility. Parallel acquisition of spectral data requires multiple photodetectors, which negatively affects cost and reliability of the monitor. The respective disadvantages of the two approaches are exacerbated when higher wavelength resolution and a larger number of spectral points to be analyzed is required. For example, both the size of the dispersive element and the number of photodiodes scale proportionally to the wavelength resolution, thereby increasing the size and cost of the device and reducing it reliability. Similarly, higher wavelength resolution requires larger tunable filters and progressively more strict requirements on tuning filters wherein progressively finer tuning is required, complicating the control loops and affecting reproducibility issues.
The present invention obviates these issues by providing a solution combining the aforedescribed approaches in a way wherein each the size, the design complexity, e.g. the number of photosensitive elements, and the control complexity of the monitor scales sub-linearly with a number of monitored wavelengths within a monitored range of wavelengths, thereby enabling monitoring of a large number of wavelength in a compact relatively inexpensive device which can be fabricated in a planar lightwave circuit (PLC) chip. The solution employs a multi-input multi-output dispersive element that allows avoiding the use of tunable filters that may require complex real-time monitoring and control.
One spectrum analyzer described in U.S. Pat. No. 5,617,234 issued Apr. 1, 1997 in the name of Koga et al. discloses a multi-wavelength simultaneous monitoring circuit capable of precise discrimination of wavelengths of a wavelength division multiplexed (WDM) signal, and suitable for optical integrated circuits. The device proposed by Koga is an AWG that has a single input port and multiple output ports and has photodetectors coupled to the output ports of the AWG. Although Koga's device shares certain similarities with the invention described hereafter, it requires an AWG having a number of output ports equal to a number of monitoring channels with frequency resolution better than spacing between the channels, and a number of costly photodetectors equal to the number of channels to be monitored.
An AWG has a functionality of splitting an input signal into several output frequency bands, each having a bandwidth b, centered at a set of frequencies fn spaced by an output frequency spacing Δf>b, and dispersing them in space to different locations where they are picked up by output waveguide to be output through their respective output ports. AWGs offer several advantages when used as the dispersive elements, such as compactness, option of on-chip integration with other optical components of an optical circuit thereby drastically lessening optical losses and reducing cost and complexity of the optical circuit, and manufacturing technology amenable to mass-production. However, they typically offer only limited frequency resolution with a limited, typically between 8 and 40, number of output channels, with a typical frequency spacing between output channels of ˜400 to 50 GHz. Decreasing the frequency spacing further below 25 GHz requires progressively larger and more expensive devices, with increasing cost per monitored channel.
A US patent application 2004/0096151 A1 to Svilans et al. assigned to JDS Uniphase, the assignee of this application, discloses an AWG-based OPM that combines a single-input port AWG with a tunable filter having a bandwidth and an FSR to obviate the aforementioned problems, by monitoring a larger number of channels, greater than a number of AWG output ports and associated photodiodes. The tunable filter pre-selects periodic subsets of channels to be input through the single input port of the AWG, and different subsets of channels are sent sequentially to the input port of the AWG thereby time-sharing the AWG and associated photodiodes coupled to the output ports of the AWG to acquire information about a number of spectrally-resolved channels larger than the number of AWG output ports and coupled to them photodiodes.
Although the devices proposed by Svilans et al. functions somewhat similar to the invention described herein, it employs a tunable filter that may require real-time monitoring and relatively complex control circuitry to ensure wavelength tuning reproducibility.
An object of this invention is to provide an optical performance or optical spectrum monitor that requires fewer detectors and fewer sequential acquisition events than frequencies to be monitored without the use of tunable frequency-selective elements.
It is a further object of this invention to provide an optical performance monitor that is substantially integrated within a single chip and wherein detectors are time-shared between the signals to be analyzed.
It is a further object of this invention to provide a switched optical performance monitor having a dispersive element with switchable multiple input ports wherein frequency resolution of the OPM is increased by switching between the input ports.